1. Field of the Invention
The present invention relates to a photonic switching device for switching data taking the form of digital optical signals constituting optical packets.
2. Description of the Prior Art
An optical packet is typically a protocol data unit (PDU) which, in the context of the present application, can be either fixed or variable. Consequently, this does not imply any restrictive hypothesis as to the format of the optical packet and the manner in which the packet is filled.
An optical packet arriving at an input port of a switching matrix contains routing information that can be used to route the packet within the switching matrix to one or more output ports.
A photonic switching matrix includes a plurality of input ports and a plurality of output ports. Thus it can happen that optical packets from a plurality of input ports must be switched to the same output port. Thus a plurality of optical packets intended for the same output port can arrive simultaneously, creating a conflict phenomenon known as contention.
In the prior art, this contention problem is solved by using buffer memories. Thus buffer memories can be disposed within the switching matrix. In this type of switching matrix, which is provided with a centralized buffer memory, the optical data packets supplied by the input ports are stored as they arrive and are then supplied to the output ports one by one. The buffer memory is controlled as a function of routing information indicating for each data packet to be switched the output port of the matrix to which that data packet is intended. Control means select the storage time in the buffer memory to constitute a queue for each output, avoiding conflicts between two data packets to be switched to the same output and reaching the input of the switching matrix at the same time.
While it is being routed between an input port and an output port, the optical data packet is therefore routed to an optical buffer memory common to all the outputs of the matrix, in which it is stored for a time period that can be selected between 0 and K.Tc, K being an integer and Tc being the duration of a data packet. The duration of a data packet is the time necessary to transmit a data packet to an output port. A buffer memory consists of optical delay lines that supply data packets staggered in time to the same output port.
However, this type of architecture, in which contention is resolved within the matrix itself, has a number of disadvantages. In particular, in this type of architecture all the traffic enters the memories. Now, buffer memory being a rare resource in optics, this architecture is deficient in terms of statistical division multiplexing. Moreover, consuming buffer memory resources, the prior art architecture described above is costly and complex to implement.
There are also architectures in which the buffer memory is external to the switching matrix.
The invention relates to this latter type of architecture, in which the buffer memory is external to the switching matrix.
In prior art architectures of this type, the buffer memory is shared for all the traffic in conflict. Thus only the traffic in conflict uses the buffer memory resource. Also, some output ports of the switching matrix are specifically dedicated to buffer memory access.
With this type of architecture a number of ports dedicated to buffer memory access are added and, when contention occurs, the optical packets that can be switched are sent over the normal lines.
Consequently, the size of the matrix is increased to enable sufficient buffer memory access whilst retaining acceptable performance in terms of the average bit rate at each input port.
One of the major disadvantages of this type of architecture, with a buffer memory external to the matrix and dedicated ports for buffer memory access, is therefore that it requires an increase in the size of the matrix of the order of 20 to 25% to achieve correct switching performance.
Also, the problem that the invention proposes to solve is that of using a photonic switching matrix in which there is no longer any contention between the optical packets, whilst alleviating the disadvantages of the prior art described above, which relate in particular to the use of the buffer memory resource and to the size of the switching matrix. The object of the invention is therefore to propose a photonic switching matrix that is simpler and less costly than prior art matrices.
To this end, the invention starts from the observation that no more traffic can leave a node than enters the node. Accordingly, when contention occurs, there are as many free output ports as there are optical packets in contention. The invention then consists in using the existing output ports to access the buffer memory. To this end, a 1-to-2 switching function is provided behind the output ports of the matrix. In this way, all the output ports of the matrix are a potential point of access to the buffer memory.